flatpak (build_manifest): some fixes needed after runtime update: - removed `--optimize=1` for lxml, as it is making the build fail - added build flag on x86_64 for Pillow (will need to be adapted if we build on other architectures) - added a module for Python 2.7.16, Python 2 not being included anymore in the runtime - DEFAULT_MANIFEST modules are put at the beginning of modules